How can I use it in a .ts file?

[sopaco]

How does it used outside the .svelte files? I have logic code in .svelte.ts and .ts files that needs to support multi-language text processing.

[gufoscuro]

Not great, but I guess you can workaround by doing

import { get } from 'svelte/store'
import { _ } from 'svelte-i18n'

function yourFunction() {
  const $_ = get(_)
  const translatedText = $_('translation-key')
}

Of course this won't be reactive, therefore if the locale changes you will need to re-execute the function to obtain the updated value

[TrapStoner]

The best way is to return the locale keys not the text itself and let the html part reactively get the proper localized text using $_ store for example:

export function formatRelativeTime(...): string {
  ...
  if (diffDays === 0) {
    return 'today'; // Return locale key 
  } else if (diffDays === 1) {
    return 'yesterday';
  } else if (diffDays < 7) {
    return get(_)('days_ago', { values: { diffDays } }); // Return localized text directly (not reactive by default you have to call your fn when locale changes to get again)
  } else {
    return formatLocalDate(updatedDate, locale, options);
  }
}

And then in the html i have

<span class="text-gray-600 text-[0.8rem]">
      {$_('updated_label')}
       {$_(formatRelativeTime(...))}
 </span>